While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. These catalysts are typically formed by reacting an organometallic complex with an activator to produce an active catalyst wherein the metal develops a positive charge and the activator develops a negative charge.
Because of the many uses of metallocenes, there has been some effort to develop zwitterionic systems wherein both the positive and negative charges are in the same compound.
Metallocycles have been prepared where the negative-charged substituent is part of the metallocycle ring such as in the structure: described by Peters et al. (J. Am. Chem. Soc. 124 (2002) 5272 and J. Am. Chem. Soc. 123 (2001) 5100) and studied for ethylene copolymerizations with carbon monoxide.
In Organometallics 22 (2003) 1503, zwitterionic zirconium complexes were used to polymerize ethylene. These complexes have the structure: with the negative-charged substituent (a tetravalent aluminum compound) not being attached to the metallocycle at either of the nitrogen atoms.
Other zwitterionic compounds, where the negative-charged substituent is attached to the metallocycle at a point other than at one of the heteroatoms chelated to the metal, and their use in ethylene polymerizations include: as reported in J. Am. Chem. Soc. 122 (2000) 1830 and: as reported in J. Am. Chem. Soc. 123 (2001) 5352, Organometallics 20 (2001) 5425 and Organometallics 21 (2002) 3082 and: as reported in Organometallics 21 (2002) 3481.
None of these metallocycles has the substituent bearing the negative charge attached to one of the heteroatoms of the metallocycle.